1. Field of the Invention
The invention relates to a plant for the production of metal melts, in particular iron melts, such as steel melts, crude steel melts or pig iron melts, and a process for the production of these melts.
2. Description of the Related Art
The present standard aggregate used for the production of electric steel is an a.c. or d.c. electric arc furnace. The iron carriers charged, which are comprised of
60 to 100% steel scrap, directly reduced iron-sponge iron in various quantitative ratios and sometimes also iron carbide (at present, up to about 10 to 20% of the total charge), and
0 to 40% liquid and/or solid pig iron
are melted by aid of one or several electric arcs using oxygen lance(s)xe2x80x94if desired, burner(s), nozzles and/or inert gas flushingxe2x80x94and under the addition of carbon carriers and slag formers. After this, the steel bath during a flat bath period (5 to 10 min) in an electric arc furnace is brought to the temperature and composition desired for tapping and is killed in the ladle during tapping. Energy and material consumption as well as plant productivity vary greatly as a function of the respective charging ratios and melting practice.
Due to world-wide introduction of secondary metallurgical processes as well as a series of developments on the constructive, electric and technological sectors, electric arc furnace melting has changed within the past few years into a process both flexible and efficient in terms of charging substances and steel quality produced, more and more exhibiting substantial advantages over converter metallurgy and competing the same successfully. With new process developments, important reductions of the melting time and the specific electric energy consumption and hence further reduction of the specific operating and investment costs of electric steel production in electric arc furnaces have been attained primarily by applying
integrated scrap preheating and/or hot charging of sponge iron/hot briquetted directly reduced iron
continuous addition of a major portion of the charging substances (iron carriers, carbon carriers, fluxes, etc.) while minimizing the power-off time for carrying out charging operations
optimum foamed slag operation and
cheaper primary energies (coal, natural gas, etc.) as a substitute for electric energy, including an improved offgas-afterburning operation and more efficient utilization of heat.
However, with the known processes for the production of electric steel by means of electric arc furnaces used as melting aggregates, the potential advantages of the above-mentioned process developments have been utilized to a limited extent only. Moreover, it has not been feasible so farxe2x80x94despite an increasing demandxe2x80x94to process to liquid steel high portions of liquid pig iron and/or other carbon-rich iron carriers (sponge iron, iron carbide etc.) as well as problem scrap (used cars) of about 30 to 70% charged into electric arc furnaces, at a high productivity and energy utilization and, with car scrap, also without inadmissible loads on the environment. The commercial application of a technology and plant based on electric arc furnace technology and highly efficient under such conditions from an economic point of view is still wanting.
The above-mentioned limitations with conventional electric arc furnaces are due exclusively to the configuration of the furnaces, which does not enable a quasi-stationary continuous process course. The operations of charging, melting, refining, heating and tapping take place on one site, by necessity more or less offset in time and with interruption(s) of the charge and current supplyxe2x80x94at least before and after tappingxe2x80x94in order to obtain the desired composition and temperature (homogeneity and overheating in respect of the liquidus temperature) of the crude steel. The present process course in an electric arc furnace is discontinuous and hence limited in its performance. In this respect, the following is noted:
1. With already reached tap-to-tap times of xe2x89xa655 min with conventional electric arc furnaces and xe2x89xa65 min for electric arc furnaces with shaft, respectively, for tap weights of 70 to 150 tons, the possibility of further reducing the power-off phases is strongly limited. The same holds for the power-on phasesxe2x80x94since under such conditions the limits for an economic energy input per ton of charge and time unitxe2x80x94and hence for the overall melting time have almost been reached.
2. In continuous charging as well as in refining and heating in the flat bath operation, which will take a substantially longer time with high charging portions of sponge iron and, in particular, of liquid pig iron and iron carbide (about 6.1% C), thus also increasing the heat loss, the actual transformer output, as a rule, is not completely utilized by electric arc furnaces.
From AT-B-295,566 a process for the continuous production of steel by melting, prereduced ore and subsequently refining the melt of semi-steel to steel in an electric arc melting furnace comprising a melting hearth to which a refining zone and at least one slag depositing chamber are connected is known, in which prereduced iron ore is introduced into the electric arc zone of the melting hearth in a lumpy or granular form, the metal is continuously agitated and set in a circulatory movement within the hearth and the metal is refined to steel while flowing through a refining zone by blowing in an oxygen-containing gas, whereas slag is caused to stream opposite to the metal at least along part of the length of the refining zone. The slag calms down in a slag depositing chamber without intensive mixing of the bath and then is tapped from the slag depositing chamber.
In that known process, plant scrap and liquid pig iron may be charged, yet each in very limited amounts only. Discharging of the offgases takes place directly in the refining zone, i.e., not via the electric arc melting furnace. The refining zone is constructed as a channel-type reactor, resulting in a high specific surface area with high heat losses. Refining is carried out with a C-concentration gradient along the refining zone of the channel-type reactor without a concentration balancing tank, and therefore the C content is difficult to adjust or control. Consequently, that known process is applicable to a limited extent only, in the first place serving to produce crude steel from prereduced ore.
From DE-C 3 609 923 a process and an arrangement for continuously melting scrap to crude steel is known. In that process, which primarily is limited to scrap melting (no mention being made of charging liquid pig iron and/or sponge iron), the heat of the furnace gases is utilized or heating the scrap. The scrap is preheated in a shaft centrally placed on the hearth-type furnace and is introduced centrally into the hearth-type furnace, thereby forming a scrap column supported on the bottom of the electric arc furnace under formation of a conical pile and capable of reaching up as far as to the scrap charging opening provided in the upper part of the scrap preheating shaft. Pivotable electrodes (preferably four electrodes) are symmetrically arranged about the scrap column in the electric arc furnace and assist in melting the scrap. The angle of inclination between the central axis of an electrode and a vertical line during scrap melting amounts to more than 20xc2x0 for each of the electrodes. Thereby, the hearth-type furnace is exposed to a great thermal load, since the electric arcs are burning between the centrally introduced scrap column and the walls and lid of the hearth-type furnace. On the one hand, this causes an increased wear of the refractory lining and hence elevated material and time costs for doing repairs. In addition, a large portion of the input energy is imparted by radiation to the furnace walls and the furnace lid and thereby gets lost. Moreover, possible bridging within the scrap columnxe2x80x94above the melt caverns melted into it by the electrodesxe2x80x94may cause precipitation of the scrap column (or parts thereof), which might lead to a break of the electrodes and hence interrupt the process.
From MPT International 2/1996, pages 56 to 60, the Contiarc process is known in which scrap is melted continuously, namely in an annular shaft furnace. This process serves exclusively for the melting of scrap; charging of sponge iron and/or liquid pig iron are not mentioned at all. One disadvantage associated with this method are the difficulties in adjusting the crude steel temperature immediately before starting and while performing the tapping operation, since there is a very large contact area of the scrap, which is arranged in the shape of a ring, with the liquid bath. There may also arise difficulties in respect of the balancing out of concentrations or in respect of the chemical homogeneity of the melt which is refined and tapped discontinuously with this process.
According to the Consteel(copyright) process (known from Electric Furnace Conference Proceedings 1992, pp. 309 to 313), scrap is preheated using an elongated horizontal preheating furnace and is charged to an electric furnace, namely at one side of the electric furnace. The offgas arising in the electric furnace is carried off via the elongated preheater for the scrap. However, no optimum gas utilization results in this process, since the scrap is not streamed through by the offgas but the latter only passes across the same. The elongated preheating channel for the scrap is stationarily arranged, whereas the electric furnace is mounted so as to be tiltable in order to enable a crude steel tapping operation which is discontinuous with this process. The structure as such is thus expensive, as with all pivotable furnaces. There results mechanical wear of the refractory furnace lining. Charging of the scrap is discontinuous, since the scrap is introduced on one side of the furnace only, namely is deposited in a marginal region of the furnace. As a result, the melting and mixing operations cannot be carried out in an optimum manner, and in using burners in the electric furnace to support melting of the scrap only a low efficiency would be achieved. The content of dust in the offgas is relatively large since scrap is not filtered off from the offgas.
The invention aims at avoiding these drawbacks and difficulties and has as its object to provide a plant as well as a process for producing metal melts, in particular iron melts, which basically enable the charging of any metal carriers incurring in metallurgical practice, preferably iron carriers having various physico-chemical properties, such as iron scrap, liquid and/or solid pig iron, iron carbide, sponge iron, iron ore having different degrees of prereduction, sinter, scales, metallurgical dust, dried sludges, etc., in various quantative compositions such that, for instance, if there is a shortage of one iron carrier another one may be used instead without capacity restrictions.
To achieve this object, a plant according to the invention is provided with the following characteristic features: comprising
an electric arc furnace vessel provided with at least one charging opening for a metal melt and/or scrap and/or direct reduced metal, in particular direct reduced iron, and/or ore and at least one electrode as well as at least one slag tapping means,
an oxygen-blowing converter vessel provided with at least one metal tapping means, wherein
the oxygen-blowing converter vessel and the electric arc furnace vessel form a unit which is connected via an overflow weir, and
the bath surface related specifically to the bath volume is smaller in the oxygen-blowing converter vessel than in the electric arc furnace vessel and
the oxygen-blowing converter vessel shares a common reaction space with the electric arc furnace vessel, which space is arranged above the oath level of these vessels.
The plant in accordance with the invention in addition to solving the problem defined above offers the advantage that in case of continuous tapping the refractory lining of the plant parts is subjected to no and in case of discontinuous tapping only to slight strains resulting from changes in temperature.
Due to the unit composed of the converter vessel and the electric arc furnace vessel being preferably rigidly arranged with respect to the foundation there is no mechanical load on the vessels, in particular the refractory lining thereof, by tilting movements or by any weight shifts resulting therefrom. In addition, the refractory brick-lining inside the electric arc furnace vessel will be protected since, in that vessel, a metal melt rich in C at all times exerts a reducing effect on the slag or lowers the content of FeO in the slag, respectively. The temperature within the electric arc furnace vessel is relatively low, namely lower than 1600xc2x0 C.
For an optimum refining operation in the oxygen-blowing converter vessel it is of advantage if the tapping-level of the metal bath of the oxygen-blowing converter vessel is located below the level of the metal bath of the electric arc furnace vessel, wherein the bottom of the oxygen-blowing converter vessel is advantageously arranged on a lower level than the bottom of the electric arc furnace vessel.
Preferably, the oxygen-blowing converter vessel is provided with one blowing lance for oxygen or an oxygen-containing gas mixture.
According to a preferred variant, the oxygen-blowing converter vessel is provided with bottom nozzles, preferably with oxygen-blowing bottom nozzles.
Advantageously, the electric arc furnace vessel is provided with at least one metal tapping means.
Suitably, the slag tapping means is provided on a decanting vessel which forms a unit with the electric arc furnace vessel, which decanting vessel is preferably arranged so as to be located diametrically opposite the overflow weir. Hereby it feasible to make the slag forming in the oxygen-blowing converter vessel flow into the electric arc furnace vessel in counterflow to the metal melt.
Suitably, the oxygen-blowing converter vessel and/or the electric arc furnace vessel is/are provided with a charging opening for charging metallic charging substances, ore, fluxes. alloys, carburizing agents and, further, the oxygen-blowing converter vessel is provided with afterburning nozzles and/or lances feeding an oxygen-containing gas or oxygen, preferably at least one thereof in the vicinity of the transition between the two vessels.
According to a preferred embodiment, the electric arc furnace vessel is provided with at least one preheating shaft supplying solid iron carriers which is arranged above the electric arc furnace vessel and preferably at the side thereof or annularly above the furnace vessel, thus enabling preheated scrap and/or sponge iron or other iron carriers to be charged in a simple manner and while utilizing the heat content of the offgases arising in the electric arc furnace vessel. The preheating shaft may be arranged centrally or at a de-centralized position and preferably is not provided with gas-permeable shut-off devices (fingers), i.e. the preheating shaft discharges into the electric arc furnace vessel directly and without any obstacles, with the solid iron carriers forming a column having its base on the bottom of the electric arc furnace vessel.
According to another preferred embodiment, at least one conveyor belt being preferably provided with a casing enters the preheating shaft, wherein, suitably, the casing is entered by heating means mounted in the casing and configured as afterburning means and/or burners having ducts feeding an oxygen-containing gas.
For efficient use of the supplied energy advantageously at least part of the inner surface of the preheating shaft and/or the casing and/or the lid of the electric arc furnace vessel and/or the lid of the oxygen-blowing converter vessel is lined with refractory materials.
Preferably the electric arc furnace vessel is provided with a means for feeding a metal melt, preferably pig iron.
According to an alternative variant, the electric arc furnace vessel is provided with a preheating shaft, which is arranged above the electric arc furnace vessel and via a gas-permeable, cooled shut-off device opens into the electric arc furnace vessel.
An alternative embodiment is characterized in that the preheating shaft is arranged centrally above the electric arc furnace vessel and the lid of the electric arc furnace vessel is designed to be annular so as to surround the preheating shaft and connect the same with side walls of the electric arc furnace vessel, with electrodes, preferably graphite electrodes, projecting through the lid into the interior of the electric arc furnace vessel in an oblique manner.
Suitably, there are provided nozzles and/or lances and/or burners opening into the interior of the electric arc furnace vessel and connected either to a supply means for iron carriers and/or an ore supply means and/or a supply means for coal or carbon carriers and/or a supply means for slagformers and/or a supply means supplying oxygen or an oxygen-containing gas and/or a hydrocarbon supply means and/or a supply means for an inert gas.
Advantageously, nozzles and/or lances are arranged in the oxygen-blowing converter, which are connected either to a supply means for iron-carriers and/or an ore supply means and/or a supply means for coal or carbon carriers and/or a supply means for slagformers and/or a supply means supplying oxygen or an oxygen-containing gas and/or a hydrocarbon supply means and/or a supply means for an inert gas.
Preferably, the nozzles are configured as sub-bath nozzles and/or bottom flushing bricks or the lances are arranged so as to be movable, in particular pivotable and/or displaceable in their longitudinal direction.
According to a preferred embodiment, the electric arc furnace vessel is provided with (one) roughly centrally arranged electrode(s) projecting into the vessel from above as well as optionally with a bottom electrode.
To enable a wide variety of uses of the plant, the preheating shaft is preferably configured as a unit separable from the electric arc furnace vessel and from the casing and exchangeable.
For easier handling, the lid of the electric arc furnace vessel and the lid of the oxygen-blowing converter vessel form a unit or are configured as a unit.
Suitably, there is provided at least one control and/or repair opening, preferably above the transition from the electric arc furnace vessel to the oxygen-blowing converter vessel.
To avoid major interruptions when individual plant parts are in need of repair, an advantageous embodiment is characterized in that the oxygen-blowing converter vessel is constructed as a structural unit separable from the electric arc furnace vessel and exchangeable.
Preferably, the electric arc furnace vessel is provided with a bottom downwardly inclined in the direction towards the decanting vessel and merging into a roughly horizontally located bottom part of the decanting vessel, with the lowermost point of the bottom being provided in the decanting vessel and a metal tapping means being provided at the lowermost point of the bottom of the decanting vessel.
A process for the production of metal melts, in particular steel melts, such as crude steel melts is characterized by the combination of the following process steps:
in the electric arc furnace vessel, a pre-melt is produced and brought to a predetermined temperature level and a predetermined chemical composition,
the pre-melt flows into the oxygen-blowing converter vessel via the overflow weir continuously and irreversibly,
the pre-melt is continuously refined in the oxygen-blowing converter vessel, preferably to crude steel and
the refined melt is carried off the oxygen-blowing converter vessel continuously or discontinuously,
the slag forming in the oxygen-blowing converter vessel in counterflow flows into the electric arc furnace vessel, from which it is withdrawn.
Suitably, prefining is carried out in the electric arc furnace vessel and final refining of the metal product in the oxygen-blowing converter vessel.
Preferably, in the oxygen-blowing converter vessel a chemical composition and a temperature of the metal melt are adjusted in a continuous manner which correspond to the chemical composition and temperature of the final melt or of the end product desired for tapping.
For adjusting a high melting efficiency, if is of advantage if the offgases formed in the oxygen-blowing converter vessel are withdrawn via the electric arc furnace vessel, with CO+H2-afterburning being carried out both in the oxygen-blowing converter vessel and in the electric arc furnace vessel, wherein suitably the offgases arising in the electric arc furnace vessel and the offgases flowing over into the electric arc furnace vessel from the oxygen-blowing converter vessel are employed for preheating the lumpy charge material charged into the electric arc furnace vessel.
For better utilizing the energy, the offgases employed for preheating are afterburned step-by-step during the preheating process.
Preferably, a negative pressure is maintained in the electric arc furnace vessel and in the oxygen-blowing converter vessel.
An alternative advantageous process for the production of pig iron melts is characterized by the combination of the following process steps:
to the electric arc furnace vessel, pig iron is charged in liquid form and is brought to a predetermined temperature level,
Si- and P-contents are lowered during prerefining in the electric arc furnace vessel,
the liquid pig iron flows continuously into the oxygen-blowing converter vessel via the overflow weir,
the liquid pig iron is furthermore partially refined in a continuous manner in the oxygen-blowing converter vessel as well,
the partially refined pig iron is drawn off the oxygen-blowing converter vessel discontinuously or continuously and
the slag forming in the oxygen-blowing converter vessel flows in counterflow into the electric arc furnace vessel, from which it is withdrawn, wherein the partially refined (pretreated) pig iron suitably is finally refined to a liquid end product by conventional methods, without or with feeding of other iron carriers, in a converter or electric arc furnace provided in addition to the plant.
Preferably, the metallic charge mix is formed from at least one of the following components
scrap, such as steel scrap, and/or solid pig iron or cast iron,
direct reduced iron in the form of pellets and/or briquettes and/or iron carbide,
liquid pig iron.
For the production of alloyed steel melts or special steel melts or stainless steel melts, the metallic charge mix is formed at least from alloyed steel scrap and liquid and/or solid alloying agents and/or ferroalloys.
Preferably, the steel melt tapped from the oxygen-blowing converter vessel is subjected to further treatment as a pre-melt in a subsequent secondary metallurgical treatment including decarburization, either with or without negative pressure (vacuum). The vacuum treatment can be carried out in a VOD, RH-OB or KTB plant. The pre-melt already exhibits a C content in excess of that demanded for the quality that is to be produced.
In case the C content after treatment in the oxygen-blowing converter vessel is already as low as that desired for the final melt, the steel melt tapped from the oxygen-blowing converter vessel is subjected to further treatment as a final melt in a subsequent secondary metallurgical treatment, f.i. in a ladle furnace or a flushing unit.
In order to avoid skulls due to the slag and to be able to carry out a quantity control in respect of the slag, a liquefying or reduction treatment, respectively, of the slag is carried out in the oxygen-blowing converter vessel after predetermined process times.